My Tohatsu 8hp outboard has an 80w charging system, which I discovered did not have a voltage regulator after the installation of my solar charging system. After some research I found this was standard on most small 2 stroke outboards.  It seems that the outboard manufacturers assumed that anyone using a small outboard for charging would probably never have fully charged batteries, so the batteries would always accept the charge, and voltage regulation wasn’t important. Probably a fairly reasonable assumption unless you have some other chargeing system onboard like I now do.

I discovered the lack of voltage regulation when trying to top off the charge of my secondary battery with the engine before combineing the batteries as the primary was already topped off from the solar panel, and I didn’t want to charge one by discharging the other. After about 10 minutes of running the engine at crusing RPMs I checked the battery voltage at the terminals with a multimeter, and found it was over 16v!

The charging system works off of a dedicated charging coil under the flywheel, that based on engine speed produces a AC current up to 40v at full throttle, and a rectifier converts the AC current into DC current at (roughly) half the AC voltage. That means at full throttle the engine will be putting out roughly 20 volts, way too much voltage if you don’t have a half charged battery hungry to absorb the power.

With the help of George, aka ThatBoatGuy, a moderator at, I found a combination rectifier and regulator that was meant for a Briggs & Stratton engine, model 797375, and confirmed the wireing of my engine, and how it install it.

Since the regulator only has three 16ga wires, two yellow AC Input wires, and one red DC output wire, installation was pretty simple.

model 797375

The first thing I did was remove the old rectifier to determine how it was connected. Unfortunantly  I had to cut the output wire from the old rectifier as it was hard wired to a Y adapter that was used to feed the wireing harness for the engine, so my goal of having a direct part for part replacment wasn’t quite possible, but with the installation of snap plugs on the end of the cut wires would allow easy re-installation of the old rectifier if I needed.

To begin the actual installation I cut off the red and yellow connectors, leaving the bare copper wire. Since this is a marine installation not the lawn tractor this was intended for, I tinned the end of the wires with solder to help prevent corosion. Then I used Anchor adhesive lined shrink wrap butt connectors to add about 8 inches of 16ga tinned marine wire to the end of the wires and sealed the connectors with a heat gun. At the end of the extensions I installed snap plugs to plug into the existing wireing harness without having to modify anything.

I hooked everything up, routed the wires, cleaned the paint off the mounting point so it would make a good electricial connection (it grounds via the case), and bolted the regulator in place.

I started the engine to test the regulator, and no matter what RPM the engine was at, or for how long, the voltage at the batteries never got above  14.2v,  Success!

UPDATE 10/2/11: After a summer with pretty poor winds that caused us to motor a lot, this is still going great. I also found through my monitoring of the charging system that the batery voltage reading displayed by my chart plotter is not exactly accurate. It seems Garmin tried to factor in a certain degree of expected voltage drop, and thus it actually reads anywhere from 0.2v to 0.6v higher than the batteries are actually at. 

12 Responses to “Outboard Voltage Regulator”

  • Henry says:

    I enjoyed reading about your solution to a common charging problem. I am considering doing the same retrofit with a Yamaha 6hp kicker.

    Have you encountered any heating problem associated with your Briggs & Stratton rectifier/regulator? I understand that a byproduct of the voltage regulation is heat. Any insight into your experience would be helpful. Maybe since that charge amperage is 6amps maximum it is not an issue? But it is a tight and relatively enclosed space where the rectifier/regulator would be mounted.



    • Henry,

      I have not noticed any problems with it, and I’ve motored for over 30 nautical miles in a single leg since installing it. Not to say it doesn’t get warm, but I’m not sure that it’s really getting any warmer than the rest of the engine does. I’m quite happy with how it’s worked to date.

      The ideal setup would of course be to take the rectifier output and put it into a proper charge controller, but in doing research I couldn’t find anything that fit the budget and I was confident would be reliable. This solution has served the purpose quite well.

      • chip goff says:

        Hi Brian,
        Hope this message satisfies protocol and finds you in good health. I’m a first-timer to your site, without having registered, and am not sure if I can even contact you this way.
        Assuming you do receive this, my inquiry concerns your regulator/rectifier replacement in your Tohatsu. I’m doing the same upgrade to a 9.9 Evinrude with a 5 amp max output charging system. I’ve installed a new battery which is fully charged. I prefer to keep it fully charged between uses, so any unregulated in-use charging would be pumped into an already fully charged battery. I intend to preserve this new battery as long as possible with proper maintenance and preventive steps such as this regulated charge. It’s equipped with a stock rectifier only which appears to be a nearly identical setup to your Tohatsu. The Evinrude’s stock rectifier is a FOUR WIRE piece. Studying the photos, I believe the Tohatsu original rectifier was also a FOUR WIRE unit. The Briggs replacement rectifier/regulator is a THREE WIRE unit. Is the only difference between a 3wire and 4wire the black, fourth lead used for grounding? I’m guessing the 3wire unit simply grounds through it’s case, while a 4wire piece employs a seemingly somewhat more reliable grounding lead. If this is, indeed, the case, the exact same Briggs-intended unit you prescribe should provide the same solution for my Evinrude as it has for your Tohatsu, agreed?
        Through the process of thinking this matter over while writing this note, I’ve pretty well convinced myself through the evidence at hand that this WILL safely work. But if you DO receive this inquiry and would be so kind as to respond, I’d love to hear your thoughts on the probability of it’s success. I look forward to hearing any insights, references, corrections, or criticisms you care to share on the subject (and on my conclusions) that might help steer me toward the best solution.
        Thanks so much,
        Warren (Chip) Goff

      • Chip, You are exactly correct that the 4 wire units use the 4th wire for a dedicated and potentially more secure ground (it does at least give more mounting options). The 3 wire units ground through the case of the rectifier / regulator and must be bolted to a clean piece of metal with a good ground contact. I had to clean off the paint on the bracket where I bolted mine to in order for it to work correctly. Good luck and sorry for the slow response, I wasn’t checking the blog in the winter (too much time skiing!)


  • Eric Johnson says:

    Thanks for this write-up. I have a nearly identical situation with my 5HP Tohatsu on my sailboat. I recently removed the battery and found a lot of the cells nearly dry. I am not certain if that was from the motor, my 5W solar panel (without a controller) or other means of charging, or just age (it was 8 years old).

    In any event, I’m trying to get my electrical house in order. I’ve added a charge controller to my solar panel. Then I stumbled upon your site while looking for ways to keep the motor coil from over-charging. That sounds like a real smart solution. Still happy with it? Is there anything magic (about fit, specs, or operation) about that particular regulator/rectifier? Or will any similar regulator/rectifier likely work? I see there are many available for lawn equipment, and also motorcycles/ATVs.

    • Still happy about it, have not had any issues with over voltage since installing it. Nothing particularly magic about that specific regulator, but it does fit on the mount as shown, where anything larger wouldn’t, and it is reasonably priced, so I would say it isn’t worth much effort to find any other model since this one both fits and works!

  • Adam says:

    I would not leave it in the engine compartment! I would detinately mount it somewhere externally. You are asking for diasater.

    • In what way is that asking for disaster? Voltage regulators are normally part of most charging systems, and the charging systems are typically (almost exclusively) in the engine compartment. Additionally the rectifier that was removed, came from the engine compartment, and that was put there by the manufacturer. There is no more risk having this in the engine than there is already present simply by having a charging system at all.

  • william says:

    i tried this on a 1990 evinrude 60 hp. works great while idling but at higher rpm’s the problem returns, high voltage.

    • I haven’t had a single high voltage issue with the engine since doing this upgrade. I have a voltage alarm set on the chart plotter that sounds at 14.5v, and has never gone off.

  • william says:

    can you think of any reason mine would be differant ? i want this to work so any help would be great.

    • Given that your engine is largeer, and presumably has a much larger charging coil to match, it may just be that this regulator cannot handle that much amperage… you probably need a regulator designed for a engine that size. The one I used was made for a small garden tractor at 10-20hp maximum…

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